The present invention relates to fiber optic communication systems and devices, and other optical systems, and more particularly, to wavelength division multiplexing (WDM) for fiber optic communication, and to a system which enables the introduction of many wavelengths in fiber optic communication systems, by use of a tunable source which enables switching from wavelength to wavelength to be able to choose different channels in communication networks.
Fiber-optic communications is a most important method of communications. A large amount of communication is transferred via fiber optics systems. Examples of such communications beyond telephones are Internet and cable TV networks. The present commercial transmission rate of a single fiber with a single wavelength can reach as high as 2.5 Gb/sec or 10 Gb/second. A way to expand the transmission capacity is by the use of more than one wavelength on a single fiber. This technology is called WDM.
The state of the art systems allow more than 32 channels per fiber, and this capacity is expected to increase. Most of today""s communication systems are known as passive systems, meaning that the particular wavelengths are assigned to particular addresses and there is a limited ability to change these dynamically during the communication. Dynamically tunable light sources and systems exist, however today they are limited.
Presently, solutions for such needs are mainly based on semiconductor laser arrays and waveguide grating technology, as described in the paper by U. Koren, Optical Fiber Communication Conference, Vol. 2 of 1998 OSA Technical Digest Series (Optical Society of America, Washington D.C. 1998) p.376, the paper by M. Zirngibl, C. H. Joyner, L. W. Stulz, U. Koren, M. D. Chien, M. G. Young and B. I. Miller, IEEE Photon. Technol. Lett, Vol.6, p.516, (1994) and the paper to M. Zirngibl, C. H. Joyner and L. W. Stulz, Electron Lett., Vol. 30, p.1484, (1994). The devices described are subject to bottlenecks in their switching capabilities, and in their multiwavelength dynamic and routing capabilities.
A direct use of fiber components and fiber lasers for such purposes, is more complicated, since straight modulation and high speed control of the gain medium are neither a simple nor a practical option. Then, a simple controllable multi-wavelength operation can be mainly based on tunable filters, such as temperature or strain dependent fiber gratings, which can be switched only at a relatively slow speed. The switching time scale of current systems reaches only the millsecond range.
Therefore, it would be desirable to provide a system and method for high speed wavelength selection in a single laser source to enable its use in dynamic wavelength division multiplexing communication methods and other systems.
Accordingly, it is a principal object of the present invention to overcome the limitations of prior art wavelength switching systems and provide a system that enables multi-wavelength selection in laser sources, enabling their use in the communication network in a special way. The most common laser for communications is the indium phosphide semiconductor laser and there are several versions of this device. Also used are erbium-doped fiber lasers based on fiber optic technology. The desire is to take the lasers and operate them at multiple wavelengths, and to be able to select and switch from the multiple wavelengths available.
According to a preferred embodiment of the present invention, there is provided a wavelength-selectable laser system comprising:
a complex laser cavity defined by a reflective mirror at one end and at the other end, a series of spaced apart wavelength-selective reflective mirrors each defining a respective predetermined cavity length;
a laser gain element; and
an active modulation element in said complex laser cavity operable to selectively provide mode-locking of a specific cavity length among said defined predetermined cavity lengths.
The goal of the present invention is to operate a laser at particularly chosen wavelengths with the ability to switch from wavelength to wavelength since the laser can operate at more than one wavelength. The method is based on cavity-resonance-operated wavelength selection (CROWS). It is operated by applying the suitable mode-locking frequency of a multiple-length fiber or diode laser cavity, formed for example, by successive multiple fiber grating reflectors, each with a different Bragg wavelength. It allows selection and high speed switching between these wavelengths. This can be useful for dynamic wavelength division multiplexing (WDM) in fiber optic communication systems having multiple wavelength channels in a single fiber, each channel carrying information which can be routed and processed independently.
Usually a laser cavity is constructed using two end mirrors. One can be a broadband mirror at a first end. In the present invention, at the second end there are located a plurality of selective mirrors one after the other in different locations and these define different lengths of the cavity. A parallel geometry for gratings is also possible, but it introduces losses due to splitters which might be needed. As a result, every cavity length has a different mode-locking frequency. Different cavity lengths in turn define different wavelengths of the laser operation.
To select the wavelength of operation of the laser, there is chosen a mode-locked frequency that is associated with the length of this cavity. If a specific wavelength is desired in the laser, the mode-locked frequency is chosen and this implies a choice of the selective mirror which is associated with that wavelength. Switching to a different wavelength is simply achieved by applying a different mode-locked frequency which is matched to the cavity length that forms this selective mirror or grating. This grating provides the new wavelength which is chosen.
The present invention features high speed wavelength tuning reaching the nanosecond switching time range. The system also features compactness, simple construction and low cost, and employs reliable fiber optic technology which is easily integrated and connected to fiber optic systems. The system is scalable to a large number of wavelengths.
Other features and advantages of the invention will become apparent from the following drawings and the description.